Microsoft is a company with a very diverse product spectrum and $60B in revenue. One has to give them some credit for sustaining high level of revenues and profit growth. It would be only fair to consider that they do have a lot of very smart people who can perhaps tell when is a short cut (FAST, Powerset ?) a better way to success and when the sheer commitment to high level of craftsmanship is the primary way.
With the search business being so dominated by Google (and for very good reason), introducing an entirely new brand name like Bing is a LOT of work, just to get people's ears and eyeballs. To overcome such a huge startup latency because of a new brand, a $100m investment in marketing sounds perfectly reasonable. Overall, I think they are investing a total of ~$500m in the online business operating expenses for the year.